Distributed feedback laser with differential grating

ABSTRACT

This disclosure concerns distributed feedback (“DFB”) lasers. In one example, a DFB laser includes a body that has first and second end facets. The DFB laser is implemented in a stack configuration that includes an active region interposed between a first top layer and a substrate. A second top layer is disposed on the first top layer and has an index of refraction different from that of the first top layer. Additionally, a grating is defined in one of the top layers and extends from the first end facet to the second end facet. The grating includes a tooth/gap structure whose configuration varies between the first end facet and the second end facet. Finally, an antireflective (AR) coating is disposed on the first end facet and on the second end facet.

RELATED APPLICATIONS

This application is a divisional, and claims the benefit, of U.S. patentapplication Ser. No. 10/284,128, entitled DISTRIBUTED FEEDBACK LASERHAVING A DIFFERENTIAL GRATING, filed Oct. 30, 2002, incorporated hereinin its entirety by this reference.

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present invention generally relates to semiconductor laser devices.More particularly, the present invention relates to a distributedfeedback laser device having a structure that improves bothmanufacturing yield and operating performance of the laser device.

2. The Related Technology

Semiconductor lasers are currently used in a variety of technologies andapplications, including optical communications networks. One type ofsemiconductor laser is the distributed feedback (DFB) laser. The DFBlaser produces a stream of coherent, monochromatic light by stimulatingphoton emission from a solid state material. DFB lasers are commonlyused in optical transmitters, which are responsible for modulatingelectrical signals into optical signals for transmission via an opticalcommunication network.

Generally, a DFB laser includes a positively or negatively doped bottomlayer or substrate, and a top layer that is oppositely doped withrespect to the bottom layer. An active region, bounded by confinementregions, is included at the junction of the two layers. These structurestogether form the laser body. A coherent stream of light that isproduced in the active region of the DFB laser can be emitted througheither longitudinal end, or facet, of the laser body. One facet istypically coated with a high reflective material that redirects photonsproduced in the active region toward the other facet in order tomaximize the emission of coherent light from that facet end.

A grating is included in either the top or bottom layer to assist inproducing a coherent photon beam. For example, the grating is typicallyproduced in the top layer of the DFB laser body by depositing a firstp-doped top layer having a first index of refraction atop the activeregion, then etching evenly spaced grooves into the first top layer toform a tooth and gap cross sectional grating structure along the lengthof the grating. A second p-doped top layer having a second index ofrefraction is deposited atop the first top layer such that it covers andfills in the grating structure. During operation of the DFB laser, thetooth and gap structure of the grating, which is overlapped by opticalfield patterns created in the active region, provides reflectivesurfaces that couple both forward and backward propagating coherentlight waves that are produced in the active region of the laser. Thus,the grating provides feedback, thereby allowing the active region tosupport coherent light wave oscillation. This feedback occurs along thelength of the grating, hence the name of distributed feedback laser.After reflection is complete, the amplified light waves are then outputvia the output end facet as a coherent light signal. DFB lasers aretypically known as single mode devices as they produce light signals atone of several distinct wavelengths, such as 1,310 nm or 1,550 nm. Suchlight signals are appropriate for use in transmitting information overgreat distances via an optical communications network.

DFB lasers as described above are typically mass produced onsemiconductor wafers. Many DFB laser devices can be formed on a singlewafer. After fabrication, the DFB lasers are separated from one anotherby a cleaving process, which cuts each device from the wafer. Thiscleaving process creates each end facet of the DFB device body.Unfortunately, limitations inherent in the cleaving process do not allowthe laser device to be cut such that a precisely desired distance isestablished between the end facet and the nearest adjacent gratingtooth.

The inherent variability of the distance between the end facet and theadjacent grating tooth created as a result of cleaving can cause severalproblems. First, the end facet, especially an end facet that is coatedwith a high reflective coating, may be disposed adjacent the nearestgrating tooth such that the laser during operation will exhibit poorsidemode suppression, which in turn results in undesired opticalfrequencies being amplified within the laser device. These undesiredoptical frequencies can spoil the monochromatic nature of the DFB laseroutput and result in reduced performance for the apparatus in which thelaser device is disposed.

Other problems that can arise from the arbitrary cleaving processinclude an increased incidence of chirp and low power output from theDFB laser device. Chirp, or the drifting of the optical outputwavelength over time, is magnified by improper distances between thegrating and the high-reflective end facet caused by the cleavingprocess. Similarly, low power output is evidence of less-than-idealcleaving of the DFB laser device.

If one or more of the above-described problems is detected in aparticular DFB laser device after manufacture and testing, it often mustbe discarded, thereby lowering the yield of acceptable DFB laser devicesthat are produced from a wafer. In some cases, the percentage ofrejected devices suffering from any of the above problems can exceed 50%per wafer.

Attempts to mitigate the effects of low precision cleaving have involvedthe addition of one or more quarter phase shifts in the grating.However, the typical DFB grating has a continuous pattern over theentire wafer. This continuous pattern allows for the lithography to besimple. Yet, the installation of one or more quarter phase shiftsrequires the use of a special lithography apparatus. Additionally,special techniques are required in order to add such phase shifts. Thesespecial requirements necessarily increase the cost of production of eachDFB device.

In light of the above, it would be desirable to enable the production ofDFB laser devices where the yield per wafer is substantially increased.Further, a need exists for the DFB laser to exhibit good sidemodesuppression while limiting chirp and output power loss. Moreover, such asolution should be simply implemented, thereby limiting production costincreases.

BRIEF SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION

Briefly summarized, embodiments of the present invention are directed toa DFB laser device that overcomes the problems created by imprecisecleaving operations performed on DFB devices during their manufacture.Specifically, exemplary embodiments of the invention are concerned witha DFB laser having a differential grating configuration suitable forhigh yield manufacture and desirable operating characteristics, such asgood sidemode suppression, low chirp, and controlled reflectivity andoptical emission.

One exemplary DFB laser includes a body that has first and second endfacets. The DFB laser is implemented in a stack configuration thatincludes an active region interposed between a first top layer and asubstrate. A second top layer is disposed on the first top layer and hasan index of refraction different from that of the first top layer.Additionally, a grating is defined in one of the top layers and extendsfrom the first end facet to the second end facet. The grating includes atooth/gap structure whose configuration varies between the first endfacet and the second end facet. Finally, an antireflective (AR) coatingis disposed on the first end facet and on the second end facet.

These and other aspects of exemplary embodiments of the invention willbecome more fully apparent from the following description and appendedclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of thepresent invention, a more particular description of the invention willbe rendered by reference to specific embodiments thereof that areillustrated in the appended drawings. It is appreciated that thesedrawings depict only typical embodiments of the invention and aretherefore not to be considered limiting of its scope. The invention willbe described and explained with additional specificity and detailthrough the use of the accompanying drawings in which:

FIG. 1 is a perspective cutaway view of a distributed feedback laserdevice manufactured in accordance with one embodiment of the presentinvention;

FIG. 2 is a cross sectional view of the laser of FIG. 1;

FIG. 3A is a close-up cross sectional view of a portion of the gratingstructure shown in FIG. 2 according to one embodiment thereof;

FIG. 3B is a close-up cross sectional view of a portion of the gratingstructure shown in FIG. 2 according to another embodiment thereof;

FIG. 3C is a close-up cross sectional view of a portion of the gratingstructure shown in FIG. 2 according to yet another embodiment thereof;

FIG. 4 is a cross sectional view of a distributed feedback laser made inaccordance with one embodiment of the present invention;

FIG. 5A is a close-up cross sectional view of one portion of the gratingstructure shown in FIG. 4;

FIG. 5B is a close-up cross sectional view of another portion of thegrating structure shown in FIG. 4;

FIG. 5C is a close-up cross sectional view of yet another portion of thegrating structure shown in FIG. 4;

FIG. 6A is a close-up cross sectional view of one portion of the gratingstructure shown in FIG. 4 according to an alternative embodiment;

FIG. 6B is a close-up cross sectional view of another portion of thegrating structure shown in FIG. 4 according to an alternativeembodiment; and

FIG. 6C is a close-up cross sectional view of yet another portion of thegrating structure shown in FIG. 4 according to an alternativeembodiment.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

Reference will now be made to figures wherein like structures will beprovided with like reference designations. It is understood that thedrawings are diagrammatic and schematic representations of presentlypreferred embodiments of the invention, and are not limiting of thepresent invention nor are they necessarily drawn to scale.

FIGS. 1-6C depict various features of embodiments of the presentinvention, which is generally directed to a distributed feedback (“DFB”)laser that is configured so as to exhibit improved operatingcharacteristics. The present DFB laser further comprises a design thatenables it to be manufactured such that laser device yield per wafer issubstantially improved.

Reference is first made to FIG. 1, which shows a cutaway view of a DFBlaser device made in accordance with one embodiment of the presentinvention, and which is generally designated at 10. The DFB laser 10includes an n-doped bottom layer or substrate 12 on which a bottomconfinement layer 14 is disposed. An active layer 16, comprising aplurality of quantum wells or other similar structure, is disposed atopthe confinement layer 14 and is covered by a top confinement layer 18. Ap-doped first top layer 20 overlies the confinement layer 18. A grating22 is defined in the first top layer 20. A p-doped second top layer 24is disposed atop the grating 22. Alternatively, the first and second toplayers 20 and 24 can be n-doped, while the substrate 12 is p-doped. Acontact layer 26 for providing an electrical signal to the DFB laser 10is disposed atop the second top layer 24. The various layers describedabove extend between a first end facet 28 and a second end facet 30,partially shown in FIG. 1. FIG. 1 illustrates several basic componentsthat generally comprise the DFB laser device 10. It is appreciated thatadditional or alternative layers or structures can be incorporated intothe present laser device as will be understood by those of skill in theart.

Reference is now made to FIGS. 2 and 3A, which show the DFB laser device10 of FIG. 1 in cross section. In the illustrated embodiment, the firstand second end facets 28 and 30 are shown having an anti-reflective(“AR”) coating 32 disposed thereon. The AR coating 32, which istypically applied after cleaving, reduces reflection of internal lightwaves off of the end facets 28 and 30 during laser operation and insteadallows the light to exit the laser device 10 through the end facets.Though it allows light waves to exit out of both end facets 28 and 30,the present DFB laser device 10 is configured to direct a majority ofthe coherent light produced by the laser through only one end facet, aswill be described.

As mentioned, the grating 22 is disposed on a top surface of the firsttop layer 20. In greater detail, the grating 22 comprises a periodicseries of closely-spaced grooves that are etched or otherwise defined inthe first top layer 20. As will be seen, the grooves, when seen in crosssection, define a series of teeth 34 protruding from the first top layer20, and gaps 36 between adjacent teeth. The gaps 36 are filled with thesecond top layer 24 such that continuous contact is established betweenthe first top layer 20 and the second top layer. Definition of thegrating 22 on the first top layer 20 can be accomplished using knowngrating techniques, including electron beam lithography.

Though both the first top layer 20 and second top layer 24 are similarlydoped, each has a distinct index of refraction with respect to oneanother. As seen in FIG. 3A, which is magnified to show grating detail,the first top layer 20 has an index of refraction n1, while the secondtop layer has an index of refraction n2. This relative refractive indexdisparity is required to enable each tooth 34 to act as a feedbacksurface for reflecting light waves and enabling their coherentpropagation within the DFB laser 10, as is known in the art. Thus, thegrating serves as a boundary between two similarly doped materialshaving distinct indices of refraction. Further details concerning thegrating 22 are discussed further below.

As already described, both the first and second end facets 28 and 30 arecoated with AR coating 32. By making each end facet light transmissivevia the AR coating 32, problems that otherwise arise with respect to theimprecise distance between a respective end facet and the last tooth 34adjacent thereto is eliminated. In other words, lightwaves that wouldotherwise reflect off the high reflective end facet (as in the priorart) that is potentially not properly positioned with respect toadjacent teeth 34 of the grating 22 are not, in fact, reflected toundesirably interact with the coherent light waves within the laserdevice, but are instead allowed to pass through the anti-reflectivefacet and exit the device. In this way, any problems normally created asa result of the inherent randomness in the cleaving process that definesthe end facets 28 and 30 of the DFB device 10 are eliminated by makingeach end facet anti-reflective via the AR coating 32. This in turnimproves the yield of DFB laser devices from a given wafer whileimproving the sidemode suppression characteristics of each such device.

Notwithstanding the improvements in light emission integrity madepossible by the above AR-coated end facets 28 and 30, this by itself isinsufficient to optimize coherent light emission from the DFB laser 10.Without further modification, a laser device having AR-coated end facetswill emit approximately one-half of its coherent light through eitherfacet. This results in a significant waste of light emission.

To alleviate the above situation, the grating 22 is modified accordingto principles taught according to the present invention so as to directthe majority of coherent light emission through only one of the endfacets. This is accomplished by anisotropically altering the physicalconfiguration of the grating 22 as a function of position along thegrating length. For example, FIG. 3A, which is a close-up view of thecircled portion in FIG. 2, shows a portion of the grating 22 near alongitudinal mid-point 38 of the grating length. The grating 22 isbifurcated by an imaginary bifurcation line at the mid-point 38 whereineach half of the grating length defines a distinct tooth and gapstructure. On the left side of the mid-point 38 as seen in FIG. 3, afirst half 22A of the grating structure, comprising periodic teeth 34Aand gaps 36A, is substantially uniform. In contrast to this, a secondhalf 22B of the grating 22, beginning at and continuing to the right ofthe mid-point 38, is characterized by a second order tooth structure,wherein every second tooth 34B that would otherwise be present (shown indashes) is missing, and in its place a gap 36B having twice the normallength is disposed. This second order structure shown in FIG. 3Acontinues from the mid-point 38 along the entire length of the secondgrating half 22B to the second end facet 30 shown in FIG. 2. Thus, anon-uniform tooth and gap structure is established along the length ofthe grating 22.

Because of the non-uniform grating structure along the length of thegrating 22, the reflectivity per unit length of the grating, or kappa(“κ”), which is related to the particular configuration of the grating,is also non-uniform along the grating length. In the present embodiment,κ is high on the uniform first grating half 22A, and relatively lower onthe second grating half 22B. Because κ is directly related to the numberof times a light wave will be reflected from the surfaces of the gratingteeth, a lower κ number associated with the second grating half 22Bindicates that light waves will be reflected by the grating less thanthose waves traveling through the first grating half 22A, whichpossesses a higher κ value. This is so because of the particular toothand gap structure of each grating half 22A and 22B. In the first gratinghalf 22A, for instance, a propagating light wave created in the activeregion 16 will encounter a tooth/gap interface, and thus a reflectiveopportunity, at every interval “a,” corresponding to the repetitiveperiod of the teeth 34 and gaps 36. On the other hand, a light wavepropagating through the second order structure of the second gratinghalf 22B encounters a tooth/gap interface only half as many times as infirst half 22A. Thus the light wave is reflected fewer times, which inturn increases the number of light signals that are able to progress toand pass through the second end facet 30. Correspondingly, because alight signal passing though the higher κ value first grating half 22Aencounters more reflective opportunities, relatively fewer signals areable to reach and pass through the first end facet 28. Consequently, asubstantial majority of light waves pass through the second end facet 30when the DFB laser 10 is configured with a grating as shown in FIGS. 2and 3A.

In addition to the second order tooth and gap configuration shown in thesecond grating half 22B, the grating could be modified to alternativelyinclude a third, fourth, or higher order tooth and gap configuration, ifdesired. For instance, in a third order configuration, every third toothis missing from the grating structure. Such grating configurations canbe designed so as to achieve the desired reduction or increase in the κvalue for the particular grating portion involved.

It is noted that the bifurcated grating structure in FIG. 3A isseparated by the imaginary bifurcation line located at the mid-point 38.However, it is not necessary that the bifurcation occur at the mid-point38. Indeed, and in agreement with the teachings herein, the division ofgrating structure topology can be defined at any appropriate point alongthe length of the grating 22. Thus, in the present example the secondorder grating structure can alternatively occupy one-third of the lengthof the grating 22 nearest the second end facet 30, while the uniformgrating structure portion is defined along the remainder of the gratinglength. Moreover, the transition from uniform grating structure tosecond order structure is seen in FIG. 3A occurring abruptly at themid-point 38. However, the present invention is not restricted to such aconfiguration. Indeed, the transition from one grating structure toanother can occur abruptly or gradually, as may be desired for aparticular application. These principles explained here also apply tothe following additional embodiments as well.

FIG. 3B illustrates how the circled portion of the grating 22 in FIG. 2would look if modified in accordance with another embodiment of thepresent invention. As in the previous embodiment, the grating lengthhere is virtually bifurcated about the mid-point 38 to define first andsecond grating halves 22A and 22B. In this embodiment, as in theprevious embodiment, the first half 22A of the grating 22 has auniformly periodic length and tooth and gap configuration, wherein eachtooth 34A′ has a substantially similar width w1. The second half 22B ofthe grating, however, is modified in its per-tooth duty cycle such thateach substantially similar tooth 34B′ has a width w2 that is less thanthe width w1. This correspondingly increases the length of each gap 36B′disposed between the teeth 34B′.

In a similar manner to the previous embodiment, the gratingconfiguration shown in FIG. 3B features a reduced κ value on the secondgrating half 22B in comparison with the κ value of the first gratinghalf 22A. Specifically, the relatively skinnier teeth 34B′ of the secondgrating half 22B having width w2 are less effective at creatingreflections of light waves, and therefore allow a relatively greaternumber of coherent light waves to proceed without significant reflectionto exit through the second end facet 30. Correspondingly, the relativelywider teeth 34A′ of the first grating half 22A having width w1 causesubstantially more light wave reflection, thereby reducing the overalllight emission from the first end facet 28, as desired.

The differences in width between the teeth 34A′ and 34B′ in FIG. 3B arerelative. Accordingly, the width w1 can vary relative to the width w2 ina variety of possible configurations, in addition to those describedhere.

FIG. 3C depicts another embodiment of the present invention, wherein theamplitude or height of the grating teeth is modified in order to alterthe κ value on the grating 22. Here, the first grating half 22A featuresteeth 34A″ having a height h1 and periodic length a. The second gratinghalf 22B, disposed to the right of mid-point 38, possesses teeth 34B″having the same periodic length a, but with a relatively lower heighth2.

During operation of the DFB laser 10, the grating 22 shown in FIG. 3Coperates in a similar manner to previous embodiments in biasing coherentlight emission toward the second end facet 30. In particular, therelatively tall teeth 34A″ of the first grating half 22A possess arelatively high K value in comparison with the relatively lower-heightteeth 34B″ of the second grating half 22B. This differential in κ valuesbiases light emission toward the second end facet 30 nearest the secondgrating half 22B, as in previous embodiments.

It should be noted that in each of the embodiments depicted in FIGS.3A-3C, the periodic length of adjacent tooth/gap pairs remainssubstantially constant within the respective grating halves. Forexample, in FIG. 3B, each tooth/gap pair has the same period both on thefirst grating half 22A and the second grating half 22B. In FIG. 3A, thetooth/gap pairs that are present on the second grating half 22B have thesame period as those teeth on the first grating half 22A if the missingteeth of the second grating half are considered. The same principleapplies to the tooth/gap pairs in FIG. 3C.

It should also be noted that the designation of a particular end facetfor transmission of the majority of coherent light waves is not limitedto that described in the accompanying figures. The DFB laser 10 could bealternatively configured such that the majority of coherent light wavesexit to the left through the first end facet 28.

It is appreciated that the shape of the teeth 34 in the variousembodiments discussed herein can also vary from that depicted. Forinstance, instead of a square shape, the teeth could have rounded topsor comprise triangular shapes. Notwithstanding the shape of the gratingteeth, the present invention can be practiced as described herein.

Reference is now made to FIGS. 4-6C, which depict additional alternativeembodiments of the present DFB laser. As cross sectionally seen in FIG.4, a DFB laser device is generally depicted at 110 and comprises asimilar structure to the DFB laser 10 shown in FIGS. 1 and 2.Specifically, the DFB laser 110 comprises an n-doped substrate 112, abottom confinement layer 114, an active layer 116, and a top confinementlayer 118 disposed atop one another in a sandwich fashion. Overlying thetop confinement layer 118 is a p-doped first top layer 120 having agrating 122 comprised of closely-spaced grooves defined thereon, and ap-doped second top layer 124 overlying the grating 122. The first andsecond top layers 120 and 124 each possess differing indices ofrefraction. A contact layer 126 is disposed atop the second top layer124. First and second end facets 128 and 130, respectively, are shownhaving AR coatings 132 applied thereon. Though the details of thegrating 122 are not apparent in FIG. 4, they will be further describedbelow in connection with FIGS. 5-6C.

Reference is now made to FIGS. 5A, 5B, and 5C, together with FIG. 4, indescribing details of the grating 122. FIGS. 5A, 5B, and 5C are close-upcross sectional views of the designated circled portions A, B, and C,respectively, of the grating 122 shown in FIG. 4. The grating 122, asbefore, comprises a tooth and gap configuration defined in the first toplayer 120 having a plurality of teeth 134 and gaps 136 disposed betweenadjacent teeth. In contrast to the previous embodiments depicted inFIGS. 2-3C, the tooth period, or distance between the beginning of onetooth/gap pair and the beginning of a succeeding tooth/gap pair,designated as “a,” is varied along the grating length. For example, itcan be seen in FIGS. 5A-5C that the period a1 of the grating teeth 134Adisposed near the first end facet 128 is substantially less than theperiod a2 of the teeth 134B disposed near the mid-point of the grating122, indicated at 138. Similarly, the period a3 of the grating teeth134C near the second end facet 128 is substantially less than thoseteeth 134B disposed near the mid-point 138. Thus, the period of theteeth disposed along the length of the grating 122 in the presentembodiment continuously increases toward the mid point 138 of thegrating, and continuously decreases toward the end facets 128 and 130.The total magnitude of period change shown in these and in the foregoingand following figures is merely exemplary; indeed, the magnitude ofchange can be varied as desired for a particular application. Thecontinuously shaped tooth period of the grating 122 shown in FIGS. 5A-5Cserves to improve the power, frequency response, and chirpcharacteristics of the DFB laser 110 by reducing spatial hole burning(i.e., non-uniform light intensity within the laser). In particular,reduced hole burning can enhance the power output, as well as frequencyresponse, in a single-wavelength laser device. Additionally, becausespatial hole burning can influence the wavelength of the coherent lightwaves emitted by the laser, reduction of such hole burning via thealteration of the grating period as described above can reduce theamount of chirp produced by the laser during modulation. Because theeffects due to spatial hole burning are most evident in lasers having alarge product of K and L (the length of the laser), it is in such laserswhere the most significant reductions in chirp are anticipated. Theseprinciples can therefore be used to improve the operatingcharacteristics of the laser device.

FIGS. 6A, 6B, and 6C depict yet another alternative embodiment of thepresent invention. FIGS. 6A-6C depict the three regions A, B, and C, ofthe grating 122 shown in FIG. 4 according to the present embodiment. Asillustrated, the period of the grating teeth 134 is continuously variedalong the length of the grating 122: the grating teeth 134A′ disposednear the first end facet 128 have a relatively large period, asindicated by a1 in FIG. 6A, while the teeth 134B′ disposed near themid-point 138 are intermediately sized, as indicated by a2. The teeth134C′, disposed near the second end facet 130, possess a relativelysmall period, as indicated by a3. This configuration of the grating 122enables not only the chirp and frequency response of the DFB laser 110to be improved, but also biases the coherent light signal toward thefirst end facet 128, given the higher κ value possessed by the gratingportion having a period equal to, or nearly equal to, a1. The embodimentillustrated in FIGS. 6A-6C therefore combines principles taught inconnection with the variation of the grating teeth period with thoserelating to the variation of the K value along the grating length. Thisproduces a DFB laser device having desirable operating characteristicsthat minimize problems, such as yield or sidemode suppression, that areassociated with imprecise cleaving of the laser device end facets duringfabrication.

In light of the present embodiment, it is generally appreciated that theteachings of the various embodiments as disclosed herein can be combinedto produce grating configurations not explicitly illustrated here. Forexample, FIGS. 5A-6C illustrate a grating having teeth that continuouslyvary in period along the length of the grating. However, it is alsopossible to configure the grating such that the tooth period abruptlychanges at a specified point along the grating length, as seen in FIGS.3A-3C. Thus, these and other modifications are contemplated as fallingunder the present invention.

If desired, it is also possible for a phase shift, such as a quarterwavelength phase shift, to be added to the grating structure to furtherenhance coherent light output from the DFB laser. Such a phase shift canbe added at any appropriate location along the grating length, such asnear either end facet, or at the mid-point.

Finally, it is noted that, though the gratings discussed herein havebeen shown as primarily disposed above the active region, it is alsopossible to dispose a grating made in accordance with the principlestaught herein under the active region, such as in the laser substrate.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrative,not restrictive. The scope of the invention is, therefore, indicated bythe appended claims rather than by the foregoing description. Allchanges that come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

1. A distributed feedback (DFB) laser, comprising: a body having a firstend facet and a second end facet, and comprising: a substrate; first andsecond top layers stacked together, the first top layer having an indexof refraction that is different from an index of refraction of thesecond top layer; an active region interposed between the first toplayer and the substrate; and a grating defined in one of the top layers,the grating extending from the first end facet to the second end facetand the grating including a tooth/gap structure whose configurationvaries between the first end facet and the second end facet; and anantireflective (AR) coating disposed on the first end facet and on thesecond end facet.
 2. The DFB laser as recited in claim 1, wherein thesubstrate is n-doped and the first and second top layers are p-doped. 3.The DFB laser as recited in claim 1, wherein the substrate is p-dopedand the first and second top layers are n-doped.
 4. The DFB laser asrecited in claim 1, wherein the first and second top layers are insubstantially continuous contact with each other.
 5. The DFB laser asrecited in claim 1, wherein the active region includes a plurality ofquantum wells.
 6. The DFB laser as recited in claim 1, wherein thegrating is defined in the first top layer.
 7. The DFB laser as recitedin claim 1, wherein the grating is configured to have a first value ofreflectivity “κ” proximate the first end facet and a second value ofreflectivity “κ” proximate the second end facet, the first value ofreflectivity “κ” being different from the second value of reflectivity“κ.”
 8. The DFB laser as recited in claim 1, wherein the gratingcomprises first and second portions and is implemented such that aconfiguration of the tooth/gap structure in the first portion isdifferent from a configuration of the tooth/gap structure in the secondportion.
 9. The DFB laser as recited in claim 8, wherein the firstportion of the grating extends approximately from the first end facet toa midpoint of the grating, and the second portion of the grating extendsapproximately from the midpoint of the grating to the second end facet.10. The DFB laser as recited in claim 1, wherein one portion of thegrating differs from another portion of the grating with regard to oneor more of the following parameters: tooth geometry; and, tooth spacing.11. The DFB laser as recited in claim 1, wherein a transition betweendiffering portions of the grating occurs relatively abruptly.
 12. TheDFB laser as recited in claim 1, wherein a transition between differingportions of the grating occurs relatively gradually.
 13. The DFB laseras recited in claim 1, wherein the tooth/gap structure of the grating issubstantially symmetric with respect to a reference point.
 14. The DFBlaser as recited in claim 1, wherein the tooth/gap structure of thegrating is substantially asymmetric with respect to a reference point.15. The DFB laser as recited in claim 1, wherein at least one parameterof the grating structure varies substantially continuously between thefirst end facet and the second end facet.
 16. A distributed feedback(DFB) laser, comprising: a body having a first end facet and a secondend facet, and comprising: a doped substrate; top and bottom confinementlayers, the bottom confinement layer being disposed on the dopedsubstrate; an active layer interposed between the top and bottomconfinement layers; first and second top layers stacked together and thefirst top layer being disposed on the top confinement layer, each of thetop layers being doped and the first top layer having an index ofrefraction that is different from an index of refraction of the secondtop layer; and a grating defined in the first top layer, the gratingextending from the first end facet to the second end facet and thegrating including a tooth/gap structure whose configuration variesbetween the first end facet and the second end facet; a contact layerdisposed on the second top layer; and an antireflective (AR) coatingdisposed on the first end facet and on the second end facet.
 17. The DFBlaser as recited in claim 16, wherein a width of each tooth in at leasta portion of the grating is substantially equal to one-quarter thewavelength of the light waves emitted by the DFB laser.
 18. The DFBlaser as recited in claim 16, wherein teeth of the tooth/gap structurehave a substantially square cross-section.
 19. The DFB laser as recitedin claim 16, wherein teeth of the tooth/gap structure have asubstantially rectangular cross-section.
 20. The DFB laser as recited inclaim 16, wherein the first and second top layers are in substantiallycontinuous contact with each other.
 21. The DFB laser as recited inclaim 16, wherein the active region includes a plurality of quantumwells.
 22. The DFB laser as recited in claim 1, wherein the grating isconfigured to have a first value of reflectivity “κ” proximate the firstend facet and a second value of reflectivity “κ” proximate the secondend facet, the first value of reflectivity “κ” being different from thesecond value of reflectivity “κ.”
 23. The DFB laser as recited in claim16, wherein the grating comprises first and second portions and isimplemented such that a configuration of the tooth/gap structure in thefirst portion is different from a configuration of the tooth/gapstructure in the second portion.
 24. The DFB laser as recited in claim23, wherein the first portion of the grating extends approximately fromthe first end facet to a midpoint of the grating, and the second portionof the grating extends approximately from the midpoint of the grating tothe second end facet.
 25. The DFB laser as recited in claim 16, whereinone portion of the grating differs from another portion of the gratingwith regard to one or more of the following parameters: tooth geometry;and, tooth spacing.
 26. The DFB laser as recited in claim 16, wherein atransition between differing portions of the grating occurs relativelyabruptly.
 27. The DFB laser as recited in claim 16, wherein a transitionbetween differing portions of the grating occurs relatively gradually.28. The DFB laser as recited in claim 16, wherein the tooth/gapstructure of the grating is substantially symmetric with respect to areference point.
 29. The DFB laser as recited in claim 16, wherein thetooth/gap structure of the grating is substantially asymmetric withrespect to a reference point.
 30. The DFB laser as recited in claim 16,wherein at least one parameter of the grating structure variessubstantially continuously between the first end facet and the secondend facet.
 31. The DFB laser as recited in claim 30, wherein the atleast one parameter of the grating structure comprises a tooth period.32. The DFB laser as recited in claim 16, further comprising at leastone phase shifting tooth portion disposed in the grating.
 33. The DFBlaser as recited in claim 32, wherein the at least one phase shiftingtooth portion is disposed proximate one of the first and second endfacets.